Within the last five years, the application of molecular biological strategies and new cellular techniques to the study of the myelin mutants have made it possible to trace the pathway from an identified genetic defect to its cellular expression in the nervous system. In the sex-linked myelin mutants it has been proposed that a mutation in the gene for proteolipid protein (PLP), the major CNS myelin protein, is the likely cause of their dysmyelination. The long term goals of this project are to address this hypothesis in two of these mutants, the myelin deficient (md) rat and the canine shaking (sh) pup, and apply an interdisciplinary approach to the molecular genetic and cellular features of these disorders. Two specific aims address these goals: 1) Study the long term effect of the putative PLP mutation on oligodendrocytes and other glia in a longer- lived strain of md rat, and in patches of non-myelination in the optic nerves of aged female heterozygote md rats who have myelin mosaicism. MOrphologic, tissue culture and molecular (mRNA analysis and in situ hybridization) approaches will be used to determine glial cell survival, division and function in these two situations. In addition, study of the distension of the rough endoplasmic reticulum (RER), which is the unique hallmark of the oligodendrocyte defect in both mutants, will be carried out in cultures of the CNS. It is planned to identify the cells in culture with swollen RER using immunolabelling, and the accumulating material in the distended RER, using antibodies to the myelin and RER associated proteins. 2) Use the two mutants as recipients of grafted cells, both to study the potential for their CNS to be myelinated by oligodendrocytes with the normal PLP gene or by normal Schwann cells, and to determine the role that other glial cells and their trophic factors have on the dysmyelination. In addition, the potential for gene transfer using a retroviral vector containing sequences of the normal PLP gene will be carried out both in vitro and in vivo. Such an approach will help determine whether such gene transfer results in more global correction of the myelin disorders than that brought about by local transplantation. These studies have importance to the X-linked dysmyelinating disorder in man. Pelizaeus-Merzbacher disease. These studies will also address questions about the feasibility of long term myelination of axons in demyelinating disorders such as multiple sclerosis, and the potential for using transplanted glial cells and Schwann cells in such diseases. The basic questions which these experiments ask will therefore be of importance to human disease and of significance to neurobiology.